The present invention relates to a distortion compensation apparatus; and, more particularly, to a distortion compensation apparatus capable of effectively canceling out upper and lower side third-order distortions produced by an amplifier.
Distortions are produced when baseband signals having multiple frequency components, e.g., a WCDMA (Wide-Band Code Division Multiple Access) signal, a multi-carrier signal and the like are amplified by an amplifier in a communications system for amplifying a communications signal; and such distortions need to be compensated. Conventionally, in order to amplify these signals while suppressing distortions, a sufficiently large backoff is set in the amplifier or a distortion compensation process by way of a feed forward scheme or a pre-distortion scheme is carried out.
Referring to FIG. 6, there is illustrated a block diagram of a conventional amplification apparatus including a distortion compensation circuit employing the feed forward scheme.
In the amplification apparatus shown in FIG. 6, an input signal (a main signal) is divided into a primary signal and a subsidiary signal by a divider 1. The primary signal is amplified by a main amplifier 2 and transferred to a subtractor 4. The subsidiary signal is provided to the subtractor 4 through a delay line 3. The subtactor 4 subtracts the subsidiary signal inputted through the delay line 3 from a portion of the amplified primary signal inputted from the main amplifier 2 to extract distortion components. The extracted distortion components are fed to a distortion amplifier 5 and the amplified primary signal with distortions is transferred into a subtractor 7 through a delay line 6. The distortion components fed into the distortion amplifier 5 are amplified by the distortion amplifier 5 and then provided to the subtractor 7. The subtractor 7 subtracts the amplified distortion component provided from the distortion amplifier 5 from the amplified primary signal provided through the delay line 6, thereby outputting an amplified and compensated signal without distortion.
The signal inputted into the subtractor 7 through the delay line 6 includes distortions produced by the main amplifier 2 and the amplified distortion components inputted into the subtractor 7 from the distortion amplifier 5 corresponds to the distortions produced by the main amplifier 2. Therefore, the output signal of the subtractor 7 corresponds to a signal in which the distortions produced by the main amplifier 2 are removed from the primary signal amplified by the main amplifier 2. The divider 1 and the subtractors 4 and 7 are respectively implemented by, e.g., directional couplers.
Referring to FIG. 7, there is provided a block diagram of a conventional amplification apparatus including a distortion compensation circuit employing the pre-distortion scheme.
In the amplification apparatus shown in FIG. 7, a pre-distortion circuit 11 is installed at a front stage of a main amplifier 12. The pre-distortion circuit 11 introduces in advance pre-distortions having same amplitudes but differing in phase by 180 degrees (i.e., being in opposite phases) compared with actual distortions to be produced in the main signal by the main amplifier 12. Subsequently, the pre-distortion circuit 11 outputs the main signal containing the pre-distortions. The actual distortions produced by the main amplifier 12 and the pre-distortions introduced by the pre-distortion circuit 11 are cancelled out.
In such amplification apparatus, the pre-distortions introduced by the pre-distortion circuit 11 and the actual distortions produced by the main amplifier 12 should be wholly matched to each other in relation to the input variation of the main signal and frequency characteristics of distortions. The distortions produced in the amplified signal are caused by AM (Amplitude Modulation)-AM conversion and AM-PM (Phase Modulation) conversion.
The gain and phase characteristics of the pre-distortion circuit should be set to be ideal with respect to those of the amplifier. Since, however, the characteristics of AM-AM and AM-PM conversions are very complicated, the characteristics of such an ideal pre-distortion circuit only can be expressed by a complicated function, rendering it virtually impossible to analytically or computationally obtain the coefficients of characteristics curve.
Thus, an alternative amplification apparatus including a distortion compensation circuit employing the pre-distortion scheme has been contemplated as shown in FIG. 8.
In the amplification apparatus shown in FIG. 8, an input signal, e.g., an RF (Radio Frequency) signal, is divided into a primary signal and a subsidiary signal by a divider 21, wherein the primary signal is transferred to an amplitude/phase circuit 27 via a delay circuit 22 and the subsidiary signal is fed to an amplitude detector (envelope detector) 23.
The amplitude detector 23 detects an amplitude level (envelope level) of the subsidiary signal. The detection result of an analog signal is converted into a digital signal by an A/D (Analog to Digital) converter 24 and then provided to a table section 25.
In the table section 25, amplitude correction data and phase correction data are stored as a table for various amplitude levels in a memory (not shown) of the table section 25. The amplitude correction data and the phase correction data in the table corresponding to the detection result of the amplitude level inputted from the A/D converter 24 are read and loaded to a D/A converter 26. In the D/A converter 26, the amplitude correction data and the phase correction data loaded from the table section 25 are converted from a digital signal into an analog signal and provided to the amplitude/phase circuit 27.
The primary signal outputted from the divider 21 is delayed by the delay circuit 22 such that the input timing of the delayed primary signal to the amplitude/phase circuit 27 is synchronized with that of the amplitude correction data and the phase correction data from the D/A converter 26.
By means of such delay, amplitude distortion is produced based on the amplitude correction data corresponding to the amplitude level of the subsidiary signal and then introduced into the primary signal at the amplitude/phase circuit 27. At the same time, phase distortion is generated in response to the phase correction data corresponding to the amplitude level of the subsidiary signal and then added to the primary signal at the amplitude/phase circuit 27. Herein, the amplitude distortion and the phase distortion are provided by the amplitude/phase circuit 27 such that they can cancel out actual amplitude and phase distortions to be produced by a main amplifier 28.
That is, the table section 25 stores such amplitude and phase correction data that have been provided in consideration of the characteristics of the main amplifier 28 in terms of the AM-AM and AM-PM conversion thereof as a function of input level. Therefore, the amplitude/phase circuit 27 can produce, in response to the amplitude and phase correction data provided from the D/A converter 26, the predistorted primary signal having opposite characteristics to those to be produced by the main amplifier 28. As a result, ideal distortionless amplification can be realized over the whole amplification apparatus.
In other words, an output signal of the amplitude/phase circuit 27 is amplified by the main amplifier 28, and the actual amplitude distortion and the actual phase distortion produced by the main amplifier 28 are canceled out by the amplitude distortion and the phase distortion introduced by the amplitude/phase circuit 27. Consequently, an amplified signal without distortion is outputted from the main amplifier 28 via a divider 29.
The divider 29 divides the amplified signal inputted from the main amplifier 28 and dispatches a portion of the amplified signal to a distortion detection section 30.
The distortion detection section 30 detects distortion components left after compensation in the sample of the amplified signal provided from the divider 29 and feeds the detection result to a table updating circuit 31.
The table updating circuit 31 responsive to the detection result inputted from the distortion detection section 30 computes amplitude correction data and phase correction data, which can, e.g., minimize the distortion components remaining in the sample of the amplified signal obtained from the divider 29 and outputs the computation result to the table section 25 to overwrite the amplitude correction data and the phase correction data stored in the table section 25 with the best values.
By employing such an updating process of the amplitude correction data and the phase correction data performed by using the feedback scheme, an amplification apparatus can be effectively operated without being affected by the temperature variation or the aging effect.
However, there still remains a problem to be addressed in that distortion characteristics of an amplifier in general has a frequency-dependence.
FIG. 9 presents frequency spectrum of two main signals and corresponding distortion outputted from an amplifier in case where a main signal of a frequency f1 and another main signal of a frequency f2 are amplified by the amplifier. The abscissa represents frequency and the ordinate represents an intensity level of a signal. The distortion of the frequency spectrum shown in FIG. 9 corresponds to intermodulation (IM) distortion components including a lower side third-order distortion of frequency 2xc2x7f1xe2x88x92f2 and an upper side third-order distortion of frequency 2xc2x7f2xe2x88x92f1. Herein, the larger the numeral located immediately after xe2x80x9cfxe2x80x9d is, the higher the frequency is (i.e. f2 greater than f1).
As shown in FIG. 9, even when intensity levels of two main signals are identical to each other, there may occur a difference of xcex94IM(=Axe2x88x92B) between an intensity level B of the lower side third-order distortion of frequency 2xc2x7f1xe2x88x92f2 and an intensity level A of the upper side third-order distortion of frequency 2xc2x7f2xe2x88x92f1. The distortion compensation is evenly carried out throughout the whole frequency band. Therefore, if such xcex94IM occurs, components of the difference xcex94IM cannot be compensated and are left in the amplified signal after distortion compensation, even in the case where the pre-distortion circuit of the amplification apparatus shown in FIGS. 7 and 8 ideally operates.
Such difference xcex94IM may originate from processes other than a typical distortion generation mechanism of an amplifier. The intensity levels of the third-order distortion components typically produced by an amplifier are equal to each other at the lower side frequency 2xc2x7f1xe2x88x92f2 and the upper side frequency 2xc2x7f2xe2x88x92f1.
Several factors can be considered as the source of the described difference xcex94IM. It may be contemplated as one of the factors that distortion of a difference frequency f2xe2x88x92f1 originates from an even-order distortion caused by a transistor included in a main amplifier and the input signals of frequencies f1 and f2 are modulated by the distortion produced by the transistor, which is noticeable in case that a change of a drain current is large as in a class AB amplifier.
Alternatively, it may also be possible that frequencies of output components of such second harmonics as a frequency 2xc2x7f1 and a frequency 2xc2x7f2 are mixed with the frequencies of f1 and f2.
Distortion compensation problems which can take place due to the presence of the xcex94IM will now be described in detail hereinafter.
FIGS. 10A and 10B show schematic graphs for illustrating exemplary frequency spectra of the upper side third-order distortion and the lower side third-order distortion produced when a signal including frequency components f1, f2, . . . , fn (n being an integer larger than 2) is amplified by an amplifier, wherein a level of third-order distortion shown in FIG. 10A is higher than that of third-order distortion shown in FIG. 10B by X. The abscissa represents frequency and the ordinate represents a level of a signal.
In FIGS. 10A and 10B, a required level of the third-order distortion components remaining after distortion compensation is represented by dashed lines and it is required that the third-order distortion components are to be reduced below the required level by the distortion compensation. For example, in the feed forward scheme for extracting distortion from a signal amplified by a main amplifier and then canceling out the distortion based thereon from the corresponding amplified signal, if the amount of distortion cancellation is sufficiently large, e.g., greater than xe2x80x9cyxe2x80x9d shown in FIG. 10A, all the third-order distortion components shown in FIGS. 10A and 10B can be decreased below the required level. However, if the amount of distortion cancellation is xe2x80x9czxe2x80x9d, the third-order distortion components shown in FIG. 10A are left in the amplified signal by the amount of xe2x80x9cxxe2x80x9d. It can be seen from the above that in the feed forward scheme a lower level distortion component among the upper and lower side third-order distortions produced by an amplifier can be more readily compensated than a higher level distortion component thereamong and therefore the degree of precision of the distortion compensation can be improved for the lower level distortion component.
Meanwhile, in order to perform distortion compensation in high degree of precision through the use of the pre-distortion scheme, in which a predetermined distortion is introduced in advance to cancel out third-order distortion components to be produced by a main amplifier, it is needed to adjust the amplitude and phase of the pre-distortion by considering whether the third-order distortion is produced as shown in FIG. 10A or FIG. 10B.
Referring to FIGS. 11A and 11B, the above will be explained in further detail.
FIG. 11A shows a vector plot describing the distortion compensation process by the feed forward scheme. Assuming that the amount of distortion cancellation can be adaptively adjusted and the third-order distortion produced by an amplifier is represented as a vector xe2x80x9cxe2x88x92axe2x80x9d, the third-order distortion can be canceled out in its entirety by deliberately introducing a distortion as a vector xe2x80x9caxe2x80x9d having characteristics opposite to those of the third-order distortion represented produced by the amplifier, as shown in case (1) of FIG. 11A. Similarly, when the third-order distortion produced by the amplifier is represented as a vector xe2x80x9cxe2x88x92axe2x88x92bxe2x80x9d, the entire third-order distortion can be canceled out by deliberately introducing the distortion represented as a vector xe2x80x9ca+bxe2x80x9d having characteristics opposite to those of the third-order distortion produced by the amplifier, as shown in case (2) of FIG. 11A.
FIG. 11B shows a vector plot in case of performing the distortion compensation by the pre-distortion scheme. Assuming that the amount of pre-distortion is fixed and represented as a vector xe2x80x9caxe2x80x9d and the third-order distortion produced by an amplifier is represented as a vector xe2x80x9cxe2x88x92axe2x80x9d, the totality of the third-order distortion can be canceled out as shown in case (1) of FIG. 11B. However, when the third-order distortion produced by the amplifier is represented as a vector xe2x80x9cxe2x88x92axe2x88x92bxe2x80x9d, the third-order distortion component in the amount represented as a vector xe2x80x9cxe2x88x92bxe2x80x9d remains left behind as shown in case (2) of FIG. 11B.
Referring to FIG. 12, there is illustrated a graph for illustrating distortion characteristics of an amplifier in terms of the intensity levels of the third-order distortion components as a function of the difference frequency xcex94f=(f2xe2x88x92f1) between two input signals of frequencies f1 and f2, wherein the curves W1 and W2 represent the characteristics of the upper and the lower side third-order distortion components, respectively. The abscissa represents the difference frequency xcex94f and the ordinate represents the level of a signal.
As shown in FIG. 12, there occurs an offset between levels of the upper side third-order distortion component and the lower side third-order distortion component. Also, if the frequencies f1 and f2 of two input signals are changed or the difference frequency xcex94f therebetween is varied, the levels of the upper and the lower side third-order distortion components are also changed.
In case of the distortion compensation process by the feed forward scheme, therefore, if the levels of the upper and the lower side third-order distortion components produced by an amplifier under the worst case become very large, the amount of distortion compensation required for reducing the corresponding third-order distortion components down to the predetermined distortion level (required level) also becomes large, accordingly, imposing a heavy processing burden. In principle, however, if a sufficient amount of distortion cancellation can be obtained, it is possible in the feed forward scheme to compensate distortions even if there occurs a certain unbalance in the levels of the upper side and the lower side third-order distortion components.
On the other hand, in the distortion compensation by the pre-distortion scheme introducing predetermined distortion in advance, there may occur, in addition to the problems described above, a further problem that the required amount of the distortion cancellation may not be obtained if the frequency of the signal component included in a signal amplified by an amplifier deviates from the predetermined frequency.
Referring to FIG. 13A, there is illustrated an exemplary graph for illustrating output signals from an amplifier including two signals of frequencies at fi and fj (j greater than i) and third-order distortion components thereof at frequencies of 2xc2x7fjxe2x88x92fi and 2xc2x7fixe2x88x92fj, i and j being 1 to n. For the convenience of explanation, it is assumed that the two signals whose frequencies are f1 and f2, respectively, and the third-order distortion components thereof respectively have same levels. xe2x80x9cIMxe2x80x9d refers to the difference between the level of two signals and that of the third-order distortion components. In FIG. 13A, the abscissa represents a frequency and the ordinate represents a level of a signal.
In such a case, a typical characteristic of xe2x80x9cIMxe2x80x9d as a function of the difference frequency xcex94f(=fjxe2x88x92fi) is represented as in a curve Q1 of the solid line, as shown in FIG. 13B. However, considering the realization of distortion compensation in high degree of precision, it is preferable that the IM is constant as represented in a straight line Q2 of the dashed line. In FIG. 13B, the abscissa represents the difference frequency xcex94f between two signals and the ordinate represents a level of xe2x80x9cIMxe2x80x9d.
Some of the prior art references relevant to the present invention will now be described below.
The exemplary prior art references to be described below are configured such that second harmonics are shorted at an output side of a transistor for the purpose of achieving an enhanced efficiency of the transistor itself by improving an F class operation, which differs from the distortion compensation apparatus of the present invention in terms of objects and overall configurations.
For example, a high output amplifier disclosed in Japanese Patent Laid-Open Publication No. 2000-77957 (a document 1) is shown in FIG. 14, which is configured of a Field Effect Transistor (FET) T11. At an output side of the transistor T11, a fundamental frequency matching is carried out at a fundamental frequency matching circuit M11. A second harmonic signal is shorted by a short stub B1 whose line length is xc2xc of the wavelength of the fundamental frequency, so that an impedance (phase) for the second harmonic signal can be independently set by a transmission line S11. A third harmonic signal is shorted by an open stub B2, so that an impedance (phase) for the third harmonic signal can be independently set by a transmission line S12.
Further, a high output amplifier disclosed in Japanese Patent Laid-Open Publication No. 1999-220343 (a document 2) is shown in FIG. 15, which is configured of an FET T12. At an output of the transistor T12, a fundamental frequency matching is carried out by a fundamental frequency matching circuit M12. A second harmonic signal is shorted by a second harmonic signal resonance circuit including an inductor L12 and a capacitor C11 and a second harmonic signal processing circuit having a short stub B3 and a capacitor C12, wherein a line length of the short stub B3 is xc2xc of the wavelength of the fundamental frequency. Further, as shown in drawing, a lead inductor L11, a coil (a choke coil) L13 and a DC blocking capacitor C13 are also installed at this circuit. An input matching circuit (not shown) is provided at an input side of the transistor T12.
Further, a high frequency amplifier disclosed in Japanese Patent Laid-Open Publication No. 1999-234062 (a document 3) is shown in FIG. 16, which is configured of a transistor T13. At an output side of the transistor T13, a fundamental frequency matching is carried out by a fundamental frequency matching circuit M13. A second harmonic signal is shorted by a transmission line S13 of a line length of xc2xc of the wavelength of the fundamental frequency and a capacitor C14.
Further, a power amplifier disclosed in Japanese Patent Laid-Open Publication No. 1997-36670 (a document 4) is configured of a power amplifier capable of facilitating the suppression of nonlinear distortion and the reduction of power consumption, but a second harmonic is not described therein.
As described above in the prior distortion compensation apparatus employing, for example, the feed forward scheme or the pre-distortion scheme, sufficient amount of distortion cancellation may not be obtained in case of amplifying a signal having multiple frequency components by an amplifier, because the levels of the upper and the lower side third-order distortion components may be increased due to the second harmonics of the corresponding signal or changed depending on the frequency variations of the signal components included in the corresponding signal and also by the variation of the difference frequency thereof.
It is, therefore, a primary object of the present invention to provide a distortion compensation apparatus capable of increasing the amount of distortion cancellation in terms of compensating the upper side third-order distortion and the lower side third-order distortion produced by an amplifier for amplifying a signal including multiple frequency components.
It is another object of the present invention to provide a distortion compensation apparatus capable of regulating the levels of the upper and the lower side third-order distortions to be identical from one another to thereby effectively carry out the distortion compensation by, e.g., the pre-distortion scheme.
It is still another object of the present invention to provide a distortion compensation apparatus capable of scaling down the levels of the upper side and the lower side third-order distortions produced by an amplifier to thereby effectively carry out the distortion compensation by, e.g., the feed forward scheme and the pre-distortion scheme.
In accordance with a preferred embodiment of the present invention, there is provided a distortion reducing circuit, including:
means for compensating an upper side third-order distortion and a lower side third-order distortion produced by an amplifier for amplifying a fundamental signal having multiple frequency components; and
a second harmonic reflection coefficient regulation circuit, installed at an output side of the amplifier, for regulating reflection coefficients for multiple frequency components included in a second harmonic signal to have a constant value.
A distortion reducing circuit, for suppressing the upper and the lower side distortions produced by an amplifier for amplifying a fundamental signal including multiple frequency components, in accordance with the present invention includes, at the output side of the amplifier, a second harmonic reflection coefficient regulation circuit capable of regulating reflection coefficients of various frequency components included in a second harmonic signal at a constant value.
The second harmonic reflection coefficient regulation circuit regulates amplitudes and phases of the components in the second harmonic signal, by reflection after being outputted therefrom, to be constant to be fed back into the amplifier independent of the frequencies of the components. Therefore, the levels of the upper and the lower side third-order distortions produced by the amplifier can be made to be substantially identical with each other irrespective of their frequencies, and accordingly, distortion compensation can be carried out effectively by, for example, the pre-distortion scheme.
That is, in case where an input signal (a fundamental signal) into an amplifier includes multiple frequency components (including continuous bands), a second harmonic signal of the input signal also includes multiple frequency components. Reflection coefficients of the frequency components of the second harmonic signal generally differ from each other depending on their frequencies. Since, however, the present invention regulates reflection coefficients to be substantially same for the whole frequency components included in the second harmonic signal, the upper and the lower side third-order distortions originated from reflected frequency of the second harmonic signal can be regulated to have a constant intensity level, independent of their frequencies. Thus, the distortion compensation by the pre-distortion scheme, which introduces pre-distortion components having same amplitudes and phases to be produced throughout the whole frequency range to cancel out the distortions to be produced by an amplifier, can be effectively carried out by employing the present invention. Though, in this configuration, the upper side and the lower side third-order distortion components originating from the second harmonic signal may not be reduced to zero, but the unbalance between the upper side and the lower side third-order distortion components can be removed.
In a preferred distortion reducing circuit in accordance with the present invention, the second harmonic reflection coefficient regulation circuit is implemented by a second harmonic matching circuit, which produces zero valued reflection coefficients for the multiple frequency components included in the second harmonic signal.
Therefore, the amplitudes of the frequency components in the second harmonic signal, which is outputted from an amplifier and then fed back thereto by reflection, are reduced to zero for all the frequency components by the second harmonic reflection coefficient regulation circuit. Resultantly, the levels of the upper and the lower side third-order distortions are scaled down compared to the prior art, regardless of frequencies. Accordingly, improved distortion cancellation can be achieved by the feed forward scheme and the pre-distortion scheme.
In other words, matching of the whole frequency components included in the second harmonic signal produced by the amplifier is carried out in accordance with the present invention to thereby prevent the second harmonic signal from being reflected back into the amplifier. Accordingly, the upper and the lower side third-order distortion components originating from the reflected components of the second harmonic signal can be avoided and therefore a total amount of the third-order distortions produced by the amplifier can be decreased that much in comparison with the prior art. Thus, the amount of distortion to be compensated (required amount of the distortion cancellation) in the distortion compensation process by the feed forward scheme or the pre-distortion scheme can be reduced accordingly and a burden for the distortion compensation process can also be decreased. Further, by removing or reducing the upper side and the lower side third-order distortion originating from the reflected components of the second harmonic signal, the unbalance between the upper side third-order distortion and the lower side third-order distortion can also be removed or reduced. Consequently, ideal distortion compensation by, e.g., the pre-distortion scheme, can be performed over the whole band of the input signal (the fundamental signal) and the high precision distortion compensation can be realized.
In an alternative, the distortion reducing circuit in accordance with the present invention, the second harmonic reflection coefficient regulation circuit is implemented by a second harmonic short circuit, which regulates the reflection coefficients for the multiple frequency components included in the second harmonic signal to be a constant value of xe2x88x921.
Such configuration using the second harmonic short circuit is advantageous in that it can be readily realized.
Further, in a preferred distortion reducing circuit in accordance with the present invention, an amplifier is configured by using an internal matching type transistor. A corresponding distortion reducing circuit is implemented by installing a transistor chip and a second harmonic short circuit in a transistor case.
Such configuration using the internal matching type transistor is advantageous in that a compact device can be realized with improved efficiency.
The distortion reducing circuit described above in accordance with the present invention may be used with a pre-distortion circuit or a feed forward distortion circuit in order to compensate the upper and the lower side third-order distortions produced by an amplifier.
As described above, the present invention regulates the reflection coefficients of the frequency components in the second harmonic signal fed back to the amplifier to be constant independent of their frequencies, thereby removing or reducing level shifts of the upper and the lower side third-order distortion components originating from the second harmonic signal in the amplifier and lowering the corresponding levels. Thus, compensation precision of the third-order distortion components can be enhanced.
A fundamental signal including multiple frequency components may not be of any specific type and also the number of the frequency components included in the fundamental signal need not be any specific number either, as long as it is not less than 1. In addition, the frequency components included in the fundamental signal may have discrete frequencies or continuous frequencies of certain bandwidth.
Any kinds of amplifiers can be used as an amplifier of the present invention. For instance, a group of amplifiers as well as a combination of a plurality of amplifiers can be used therefor. An amplifier for amplifying a signal including multiple frequency components is referred to as a common amplifier.
Various transistors, for example, an FET, may be used as a transistor of the amplifier of the present invention.
Assuming that frequency components included in the fundamental signal respectively have frequencies f1 and f2, the upper and the lower side third-order distortions produced by an amplifier respectively represent distortions at frequencies 2xc2x7f2xe2x88x92f1 and 2xc2x7f1xe2x88x92f2. Similarly, in case where the fundamental signal includes more than two frequency components, there occur an upper and a lower side third-order distortion per each pair of two different frequency components.
In general, due to third-order distortion components originating from a second harmonic signal in an amplifier, both a total amount of intensity levels of the upper side third-order distortions and that of the lower side third-order distortions are increased and at the same time there occurs the difference (unbalance) therebetween.
A second harmonic signal is a signal including frequency components, the frequency of each component being twice fundamental frequency included in the fundamental signal. For example, for a fundamental signal including signals whose frequencies are f1 and f2, signals having frequencies 2xc2x7f1 and 2xc2x7f2 are outputted from an output terminal of the amplifier as a second harmonic signal.
Further, it is preferable that reflection coefficients for the whole frequency components included in the second harmonic signal are identical to each other. However, the reflection coefficients may differ from each other by a certain degree acceptable in practical application and such case is also encompassed by the scope of the present invention.
The second harmonic reflection coefficient regulation circuit may be implemented by various circuits. For example, a circuit having a fixed resistance, capacitance and inductance or a circuit adaptively controllable those values can be employed.
It is preferable that the compensation precision of the third-order distortion components produced by an amplifier can reduce such third-order distortions down to zero. However, a certain amount of distortions may be left behind after distortion compensation as long as it is acceptable in practical application.
It is preferable to have, together with the second harmonic reflection coefficient regulation circuit described above, a matching circuit of a fundamental signal at the output side of the amplifier. It is also preferable to have a matching circuit for a fundamental signal and a matching circuit for a second harmonic signal at an input side of the amplifier.
The reflection coefficient can be defined as: xe2x80x9ca reflection coefficient=(a voltage of a reflected wave fed back from a load)/(a voltage of a traveling wave progressing to the load)xe2x80x9d. The absolute value of the reflection coefficient, i.e., |reflection coefficient|, is smaller than or equal to 1 (|a reflection coefficient|xe2x89xa61). The zero valued reflection coefficient represents that all traveling waves are absorbed by the load without being fed back therefrom. In other words, when reflection coefficient is zero, the traveling waves are not reflected at all, representing a matching state. A reflection coefficient of a value of xe2x88x921 represents a shorted total reflection state and a reflection coefficient of a value of +1 represents an open reflection state. A reflection coefficient of |1| represents a reflection state accompanying a phase shift. Further, 0xe2x89xa6|a reflection coefficient|xe2x89xa61 represents a mismatching state where a part of the traveling waves is absorbed by the load and the remaining part thereof is fed back to the amplifier, while |a reflection coefficient| of 1 represents that a reflected wave having a same amplitude (absolute value) as that of a traveling wave is fed back from a load.
FIG. 5A represents an exemplary circuit diagram for carrying out the matching of a second harmonic signal. In this circuit, a traveling wave of the second harmonic signal outputted from a transistor Ta is absorbed by a load through a matching circuit P and a resistor R, to thereby prevent the generation of a reflected wave of the corresponding second harmonic signal.
FIG. 5B represents an exemplary circuit diagram for illustrating a total reflection of a second harmonic signal. Tn this circuit, a traveling wave of the second harmonic signal outputted from a transistor Tb is totally reflected by a second harmonic short circuit including an inductor L and a capacitor C and then fed back to the transistor Tb. The total reflection of wave can also be accomplished by replacing the second harmonic short circuit including the inductor L and the capacitor C with a circuit including shorted transmission lines, each having a length corresponding to a half wavelength of a second harmonic wave.
A principle of the present invention will now be explained below. For the convenience of explanation, it is assumed that a fundamental signal includes two frequency components in the description below. However, the same will hold in a case where more than two frequency components are included in a fundamental signal.
For example, assuming that a distortion characteristic V0 of a transistor (amplifier) is represented by Eq. (1) and that a frequency component {A1xc2x7cos(xcfx891xc2x7t)} of an angular frequency xcfx891 corresponding to a frequency f1 and a frequency component {A2xc2x7cos(xcfx892xc2x7t)} corresponding to a frequency f2 are inputted into a transistor as a fundamental signal Vin of an input signal, third-order distortion components X1 in case of without considering harmonics are represented by Eq. (2) (see, e.g., xe2x80x9cHarmonic Feedback Circuit Effects on Intermodulation Products and Adjacent Channel Leakage Power in HTB Power Amplifier for 1.95 GHz Wide-Band CDMA Cellular Phonesxe2x80x9d, IEICE TRANS. ELECTRON., VOL. E82-C, NO. MAY 5, 1999 (a document 5)).
V0=g1xc2x7Vin+g2xc2x7(Vin)2+g3xc2x7(Vin)3+xe2x80x83xe2x80x83Eq. (1) 
and
X1=(xc2xe)xc2x7A1xc2x7(A2)2xc2x7g3xc2x7cos{(2xc2x7xcfx892xe2x88x92xcfx891)xc2x7t}+(xc2xe)xc2x7(A1)2xc2x7A2xc2x7g3xc2x7cos{(2xc2x7xcfx891xe2x88x92xcfx892)xc2x7t}+xe2x80x83xe2x80x83Eq. (2) 
wherein g1, g2, g3, are respectively coefficients; A1 and A2 represent amplitudes and xe2x80x9ctxe2x80x9d represents time.
As can be see from Eq. (2), if the amplitudes A1 and A2 of two frequency components are identical with each other, the level (amplitude) of the upper side third-order distortion at frequency 2xc2x7f2xe2x88x92f1 corresponding to an angular frequency 2xc2x7xcfx892xe2x88x92xcfx891 and that of the lower side third-order distortion at frequency 2xc2x7f1xe2x88x92f2 corresponding to an angular frequency 2xc2x7xcfx891xe2x88x92xcfx892 become equal to one another regardless of the angular frequencies xcfx891 and xcfx892. The third-order distortion components represented in Eq. (2) are always produced in a typical transistor and no measure is taken in the present invention in order to directly reduce such third-order distortion components expressed in Eq. (2).
In case where a second harmonic signal of a fundamental signal is outputted by a transistor and a reflected second harmonic signal is fed back to the transistor with frequency components included in the fed-back second harmonic signal (a reflected wave signal) being represented by B1xc2x7cos(2xc2x7xcfx891xc2x7t+xcfx861) and B2xc2x7cos(2xc2x7xcfx892xc2x7t+xcfx862), the fed-back second harmonic signal is mixed with the fundamental signal and the third-order distortion components X2 represented by Eq. (3) are produced.
X2=A1xc2x7B2xc2x7g2xc2x7cos(2xc2x7xcfx892xc2x7txe2x88x92xcfx891t+xcfx862)+(xc2xe)xc2x7A1xc2x7B1xc2x7B2xc2x7g3xc2x7
cos(2xc2x7xcfx892xc2x7txe2x88x92xcfx891xc2x7t+xcfx862xe2x88x92xcfx861)+ . . . A2xc2x7B1xc2x7g2xc2x7cos(2xc2x7xcfx891xc2x7txe2x88x92xcfx892xc2x7t+xcfx861)+(xc2xe)xc2x7
A2xc2x7B1xc2x7B2xc2x7g3xc2x7cos(2xc2x7xcfx891xc2x7txe2x88x92xcfx892xc2x7t+xcfx861xe2x88x92xcfx862)+xe2x80x83xe2x80x83Eq. (3) 
wherein B1 and B2 are amplitudes of two frequency components included in the reflected second harmonic signal and xcfx861 and "PHgr"2 represent phases of the angular frequency components thereof.
As for the third-order distortion components originating from the reflected portions of the second harmonic signal, the levels (amplitudes) and the phases of an upper side and a lower side third-order distortion component differ from one another if there exist the difference between the amplitudes B1 and B2 and that between phases xcfx861 and xcfx862, as represented in Eq. (3).
The overall third-order distortion components produced by a transistor are mainly composed of the sum of the third-order distortion components originating from the fundamental signal as represented in Eq. (2) and the third-order distortion components originating from the second harmonic signal as represented in Eq. (3).
A poor management of the reflection coefficients for the frequency components in a second harmonic signal as in the prior art normally results in nonuniform reflection coefficients. As a result, amplitudes B1 and B2 and phases xcfx861 and xcfx862 of the second harmonic frequency components become different from each other, yielding poor frequency characteristics and an unbalance between the upper side and the lower side third-order distortion. However, the distortion reducing circuit in accordance with the present invention can regulate the reflection coefficients for the whole frequency components included in the second harmonic signal to have a uniform value, so that the amplitudes B1 and B2 and the phases xcfx861 and xcfx862 of the frequency component can be controlled to respectively have same values and therefore the frequency characteristics can be improved. Implementation of a circuit for leveling the reflection coefficients of the multiple frequency components included in the second harmonic signal can be realized by, e.g., computer simulation.
In a prior art distortion compensation apparatus, frequency matching is carried out only for the frequencies in an amplification band of an amplifier, i.e., a band for the fundamental frequencies of an input signal. However, the distortion reducing circuit in accordance with the present invention performs the frequency matching not only in the amplification band of the amplifier but also in the whole frequency components included in the second harmonic signal. Thus, the level of the third-order distortion originating from the second harmonic signal represented in Eq. (3) can be reduced to zero. Accordingly, the third-order distortion components produced by a transistor can be mainly composed of components originating from the fundamental signal and thus the total amount of the third-order distortion components produced by the amplifier can be minimized due to the reduction of the third-order distortion components originating from the second harmonic signal.